THE BIOLOGY OF THE CELL SURFACE 



lent description of fertilization in the egg of Amphioxus 

 this statement is taken: "Very close to the head of the 

 spermatozoon one often sees a small intensely stained point, 

 which could very well be the centrosome, but in view of the 

 many yolk-granules in the neighborhood that have the same 

 appearance, I do not dare to decide this."^ When the egg- 

 and sperm-nuclei unite, the sperm-centres continue as the 

 cleavage centres. 



The eggs of sea-urchins, fertilizable only after complete 

 maturation, represent the fourth class to be discussed. We 

 may take the egg of Arhacia as an example. 



The normal,- living, unfertilized egg of Arbacia is of 76 

 microns in diameter and bounded by a thin elastic mem- 

 brane below which is the delicate ectoplasm. The nucleus 

 in the living unfertilized egg appears as a clear space lying 

 in any position with reference to the polar axis of the egg.^ 

 On fertilization the vitelline membrane separates beginning 

 at the point of sperm-entry where a nipple-like protrusion — 

 the fertilization-cone — from the egg-surface forms to pull 

 in the sperm-head. The surface of the egg seen under high 

 power of the microscope appears delicately crenated because 



1 Sobotta, iSgy, p. 40. 



- Normal, living, unfertilized eggs of Arhacia are spheres of 

 approximately the same size and specific gravity, each enclosed in a 

 jelly hull. They possess bright red pigment granules evenly dis- 

 tributed and never clumped. Eggs that do not satisfy these criteria 

 should be rejected for experiment especially if they color the sea-water 

 in which they lie; such discharge indicates the presence of moribund 

 or cytolyzing eggs. 



^ This axis is an imaginary line passing through the centre of the 

 egg and the point at which the polar bodies are given off. In sea- 

 urchins'' eggs the polar bodies are always given off at the point 

 opposite a tube in the jelly-hull of the egg. Because in these eggs 

 the polar bodies are usually lost before the eggs are shed, the location 

 of the tube in the egg's jelly hull is important for determining the 

 egg's polar axis. 



I JO 



